In The Dreamers when Matthew tries to convince both Isabelle and Theo to not leave him, why don't they listen to him and join the riot? Both characters are really hard to understand but their thoughts portrayed in the movie are yet so simple and intriguing.
My personal interpretation:
I don't know how Siamese twins are like to each other in the real world but in the movie the relationship between the two lead characters are shown very strange yet interesting.
After watching the movie, two things were stuck in my head about these two characters, Théo (Louis Garrel) and Isabelle (Eva Green).
1 - They both share common interest, that's passion for movies.
Through out the movie you see snippets of french classic movies (that's what I think they are.) and they role-play them and they lay bets on those movies.
2 - At any cost, they don't leave each other.
The movie made me say this line because, from beginning, they sleep nude next to each other, they bath together and even sexuality (the intercourse between Isabelle and Matthew in the kitchen.) they took liberally.
So, I'm not surprised that Isabelle did not choose to leave with Matthew. Simply, Isabelle won't either leave her twin brother or the love for movies.